In the case where hot-rolled steel plates or steel sheets are used in construction machines, industrial machines, shipbuilding, steel pipes, civil engineering, steel structures such as buildings, machinery, equipment, or the like, abrasion resistant property is required for such steel plates or steel sheets in some cases. Abrasion is a phenomenon that occurs at moving parts of machines, apparatus, or the like because of the continuous contact between steels or between steel and another material such as soil or rock and therefore a surface portion of steel is scraped off.
When the abrasion resistant property of steel is poor, the failure of machinery or equipment is caused and there is a risk that the strength of structures cannot be maintained; hence, the frequent repair or replacement of worn parts is unavoidable. Therefore, there is a strong demand for an increase in abrasion resistant property of steel used in wearing parts.
In order to allow steel to have excellent abrasion resistant property, the hardness thereof has been generally increased. The hardness thereof can be significantly increased by adopting a martensite single-phase microstructure. Increasing the amount of solid solution carbon is effective in increasing the hardness of a martensite microstructure. Therefore, various abrasion resistant steel plates and steel sheets have been developed (for example, Patent Literatures 1 to 5).
On the other hand, when abrasion resistant property is required for portions of a steel plate or steel sheet, in many cases, the surface of base metal is exposed. The surface of steel contacts water vapor, moisture, or oil containing a corrosive material and the steel is corroded.
In the case where abrasion resistant steel is used in mining machinery including ore conveyers, moisture in soil and a corrosive material such as hydrogen sulfide are present. In the case where abrasion resistant steel is used in construction machinery or the like, moisture and sulfuric oxide, which are contained in diesel engines, are present. Both cases are often very severe corrosion environments. In these cases, for corrosion reactions on the surface of steel, iron produces an oxide (rust) by an anode reaction and hydrogen is produced by the cathode reaction of moisture.
In the case where hydrogen produced by a corrosion reaction permeates high-hardness steel, such as abrasion resistant steel, having a martensite microstructure, the steel is extremely embrittled and is cracked in the presence of welding residual stress due to bending work or welding or applied stress in the environment of usage. This is stress corrosion cracking. From the viewpoint of operation safety, it is important for steel for use in machinery, equipment, or the like to have excellent abrasion resistance and resistance to stress corrosion cracking.